Hi Genesh,
Are you specifying an address in your shmat calls? Can you post code
segments doing
shmget/shmat?
max
Borse, Ganesh wrote:
Hi,
I am facing a problem of shared memory segment getting attached at
different address, when the processes using it go down and attach
count goes to zero.
This causes the problem when the processes next time come up & try to
access the addresses taken from the shared memory segment.
The scenario in brief:-
I've 3 processes using multiple shared memory segments. Shared memory
segments (shm) are also used in hierarchical fashion.
One shm is used as a high level data structure to hold pointers to
other shms.
Second shm is an array of pointers to structures (data blocks) which
will get allocated in the third shm. For the other 2 processes to
access these blocks, their addresses will be stored in this second shm
(an array).
The third shm actually holds the data as retrieved by the server
process from database. This is acting as cache.
When the third shm gets completely used, the server process creates
new shm and starts storing data into it. Once all data gets stored,
server process goes down - without removing any shm.
Next when the client processes comes up, they attach to all these shms
created by server process.
When the client process calls "shmat" on the first 3 memory segments,
shmat returns the same address which it returned to server process
(these addresses are stored in 1st shm).
But, when client process tries to attach to 4th segment (which was
created after the third data shm filled up), the shmat returns totally
different address than that of returned to the server process.
Since, the addresses stored in array shm correspond to the different
shm address, when the client process tries to access those addresses,
it crashes with SIGSEGV.
I am using SHM_SHARE_MMU flag with shmat to use the feature of ISM &
to get same address.
Any clue/ hint why only the 4th (last segment) shmat returns different
address? Is this the normal behavior?
Could I be doing something wrong only while creating this segment?
Please help.
Thanks and Regards,
GaneshB
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